Centrifugal pumps, as the name implies, employ centrifugal force to lift liquids from a lower to a higher level or to produce a pressure. This type of pump, in its simplest form, comprises an impeller consisting of a connecting hub with a number of vanes and shrouds, rotating in a volute collector or casing. Liquid drawn into the center, or eye, of the impeller is picked up by the vanes and accelerated to a high velocity by rotation of the impeller. It is then discharged by centrifugal force into the casing and out the discharge branch of the casing. When liquid is forced away from the center of the impeller, a vacuum is created and more liquid flows into the center of the impeller. Consequently there is a flow through the pump. There are many forms of centrifugal pumps, including the type used to pass both solid and liquid mixtures. These are known as slurry pumps.
Slurry pumps are designed to transport solids mixed in a fluid, normally water. Slurry pump impellers are driven by a shaft that is supported by rolling element bearings contained within a bearing housing. A stuffing box is used to seal the shaft where it enters the casing of the pump. The stuffing box may have various configurations, but typically includes multiple packing rings that are compressed by a gland to form a seal around the rotating shaft between the internal working pressure of the pump and the atmosphere. When properly adjusted, a small amount of fluid (water) will leak from the stuffing box during operation. This fluid leakage is generally useful in cooling and lubricating the dynamic interface between the rotating shaft and the packing rings. As the packing rings wear under normal operation, however, this leakage increases. This ultimately causes the leakage to be directed against the bearing housing and bearing isolation seals. The bearing housing is typically a cylindrical volume that isolates the rolling element bearings from external contaminants and that contains and provides lubrication for the bearings.
There are several different bearing housing seal assembly designs currently being employed for sealing the shafts and bearing housing assemblies for centrifugal pumps. The purpose of the bearing housing seal assembly is to prevent external contaminants from entering the bearing housing and contaminating the oil or other lubricant therein. Some designs include what are known as “lip seals,” which are commonly used in automotive applications. These designs, however, are subject to rapid wear and heat generation which can damage the shaft of the pump. Other seal designs include labyrinth arrangements having a series of internal grooves designed to contain lubricant within the bearing housing. Another form of sealing arrangement is commonly known as a bearing isolator, which is comprised of a stator and rotor arrangement which cooperate to form a series of labyrinth channels to prevent contaminants from reaching the bearing area.
Unfortunately, however, the existing designs may be defeated by the fluid volume and fluid velocity encountered when the stuffing box packing becomes worn or otherwise fails. When this occurs, water and solids are directed against the bearing seal area with considerable pressure, which may effectively “flood” the bearing housing seals and ultimately cause failure of the centrifugal pump.